Question: Simplify the following expression and state the conditions under which the simplification is valid. You can assume that $p \neq 0$. $z = \dfrac{2p^2 + 14p - 16}{-4p^2 - 60p - 224} \div \dfrac{p + 9}{p + 7} $
Solution: Dividing by an expression is the same as multiplying by its inverse. $z = \dfrac{2p^2 + 14p - 16}{-4p^2 - 60p - 224} \times \dfrac{p + 7}{p + 9} $ First factor out any common factors. $z = \dfrac{2(p^2 + 7p - 8)}{-4(p^2 + 15p + 56)} \times \dfrac{p + 7}{p + 9} $ Then factor the quadratic expressions. $z = \dfrac {2(p + 8)(p - 1)} {-4(p + 8)(p + 7)} \times \dfrac {p + 7} {p + 9} $ Then multiply the two numerators and multiply the two denominators. $z = \dfrac { 2(p + 8)(p - 1) \times (p + 7)} { -4(p + 8)(p + 7) \times (p + 9)} $ $z = \dfrac {2(p + 8)(p - 1)(p + 7)} {-4(p + 8)(p + 7)(p + 9)} $ Notice that $(p + 8)$ and $(p + 7)$ appear in both the numerator and denominator so we can cancel them. $z = \dfrac {2\cancel{(p + 8)}(p - 1)(p + 7)} {-4\cancel{(p + 8)}(p + 7)(p + 9)} $ We are dividing by $p + 8$ , so $p + 8 \neq 0$ Therefore, $p \neq -8$ $z = \dfrac {2\cancel{(p + 8)}(p - 1)\cancel{(p + 7)}} {-4\cancel{(p + 8)}\cancel{(p + 7)}(p + 9)} $ We are dividing by $p + 7$ , so $p + 7 \neq 0$ Therefore, $p \neq -7$ $z = \dfrac {2(p - 1)} {-4(p + 9)} $ $ z = \dfrac{-(p - 1)}{2(p + 9)}; p \neq -8; p \neq -7 $